numero25

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 25 (2013) 138-144; DOI: 10.3221/IGF-ESIS.25.20 141 sides of the specimen. This is important because studies can be made either of the interaction between the crack and the microstructure and of the interaction between different cracks and different crack branches. A frame of the video is taken every 30 seconds in order to measure crack length. Next, the crack length is measured through the image analysis free software ImageJ. The combination of this information with the test frequency is elaborated in graphics of crack lengths depending on the number of cycles. On the basis of this graphic, the graphic on growth rate depending on the number of cycles and/or the crack length is elaborated. It is certain that the growth rate is generally expressed in relation to the parameter K or the crack length; however, if it is expressed in relation to the number of cycles, the growth rates of the different cracks can be compared at the same point in time. R ESULTS ig. 3 shows a photograph extracted from one of the fatigue tests videos. The crack grows from a circular notch with a radius of 1 mm. The grain size is of 9.74 mm. The test is load controlled, type stress-stress (R=0.1) with a maximum stress of 45 MPa; under these loading conditions and geometry of specimen, the resonance frequency is 82.13 Hz. Fatigue life is 4.09x10 6 cycles. The left image corresponds to one of the sides of a sample and the right image corresponds to the other face. The latter is shown from an inverted angle, thus, both left cracks correspond to the same side of the notch, and similarly, both cracks growing from the right side correspond to the other side of the notch. In the video, it is possible to observe crack tip arrest at grain boundaries, as well as changes in growth direction. Figure 3 : Crack growing from a circular notch with a diameter of 2 mm within a microstructure of 9.74 mm. As mentioned above, an extract of video is taken at a specific time interval (e.g. 30 seconds) in order to measure crack length and growth rate. The number of cycles is calculated on the basis of the test frequency. The crack length is measured through the use of ImageJ. Figure 4 shows crack length in relation to the number of cycles for three cracks growing from the same notch. All of them show acceleration and deceleration patterns along their trajectory. Figure 5 shows growth rate in relation to crack length for three cracks growing in different microstructures with grain sizes of 1.41, 3.46 and 9.74 mm. There is a clear correlation between the average spacing of the consecutive crack halts and the grain size of the material. An interesting observation is that in the majority of the cases studied the cracks did not initiate at the point of maximum stress concentration. Recall that we are referring to push-pull loading here. This is surprising since the classical methods of notched fatigue limit analysis clearly indicate the horizontal symmetry axis as the initiation and propagation direction. Figure 6 shows the results of measurements of the location of the crack initiation point around the circumference of the notch for different holes radii and for several specimens with the same microstructural size of 9.74 mm. On the left, a diagram shows a polar coordinate system corresponding to a quarter of the notch and it presents the crack initiation points. The graphic on the right shows the corresponding frequency distributions. Approximately 36 measurements were made for each notch radius. The following trend appears. When the size of the notch diminishes (in relation to the microstructure) the width of the frequency distribution seems to become larger and the most frequent location of the initiation point value moves away F